static int bitCount(BitSet bs) {
  ret bs == null ? 0 : bs.cardinality();
}

static int bitCount(int i) {
  ret Integer.bitCount(i);
}

static int bitCount(byte[] array) {
  if (array == null) ret 0;
  int sum = 0;
  for (b : array)
    sum += bitCount(ubyteToInt(b));
  ret sum;
}